The mass flow rate of a fluid (defined by its average velocity multiplied by its mass density multiplied by the cross-sectional area of the channel through which the flow travels) is a measured quantity of interest in the control or monitoring of most practical and industrial applications, such as any chemical reaction, combustion, heating, cooling, drying, mixing, fluid power, etc. Generally speaking, a thermal anemometer is used to measure the mass velocity at a point or small area in a flowing fluid—be it liquid or gas. The mass velocity of a flowing fluid is its velocity referenced to standard or normal temperature and pressure. The mass velocity averaged over the flow channel's cross-sectional area multiplied by the cross-sectional area is the standard or normal volumetric flow rate through the channel and is a common way of expressing the total mass flow rate through the channel.
The thermal anemometer is sometimes referred to as an immersible thermal mass flow meter because it is immersed in a flow stream or channel in contrast to other thermal mass flow meter systems, such as those which sense the total mass flow rate by means of a heated capillary tube mounted externally to the flow channel.
The operational principles of thermal anemometers derive from the fact that a heated sensor placed in a fluid stream transfers heat to the fluid in proportion to the mass flow rate of the fluid. In a thermal anemometer, one such heated sensor (commonly referred to as the velocity sensor) is provided together with another sensor that detects fluid temperature. In the constant-temperature mode of operation, the heated sensor is maintained at a constant temperature above the fluid temperature. The temperature difference between the flowing fluid and the heated sensor results in an electrical power demand in maintaining this constant temperature difference that increases proportional to the fluid mass flow rate and that can be calculated.
Alternately, some thermal anemometers operate in a constant-current mode wherein a constant current or power is applied to the heated sensor and the fluid mass flow rate is calculated from the difference in the temperature of the heated sensor and the fluid temperature sensor, which decreases as mass flow rate increases. Thermal anemometers have greater application to gases, rather than liquids, because their sensitivity in gases is higher than in liquids.
Because the parts of the heated sensor of known thermal anemometers are not sufficiently reproducible dimensionally or electrically, known thermal anemometers require multi-point flow calibration of electrical output versus mass flow rate, usually in the actual fluid and with the actual ranges of fluid temperature and pressure of the application. For industrial applications, the heated sensor and fluid temperature sensor of known thermal anemometers typically have their respective sensors encased in a protective housing shell (e.g., thermowell or metallic tube sealed at its end, etc.). Usually, the encased heated sensor is inserted into the tip of the housing and is surrounded by a potting compound, such as epoxy, ceramic cement, thermal grease, or alumina powder.
In such a system, “skin resistance” and stem conduction are two major contributors to non-ideal behavior and measurement errors in thermal anemometers constructed in this manner. Skin resistance is the thermal resistance between the encased heated sensor and the external surface of the housing exposed to the fluid flow. The well-known hot-wire thermal anemometers have zero skin resistance, but thermal anemometers with a housing do have skin resistance. The use of a potting compound substantially increases the skin resistance because such potting compounds have a relatively low thermal conductivity.
Skin resistance (in units of degrees Kelvin per watt) results in a temperature drop between the encased heated sensor and the external surface of the housing which increases as the electrical power supplied to the heated sensor increases. Skin resistance creates a “droop” and decreased sensitivity in the power versus fluid mass flow rate calibration curve, especially at higher mass flow rates. The droop is difficult to quantify and usually varies from meter to meter because of variations both in the parts of construction and in installation. The ultimate result of these skin-resistance problems is reduced accuracy. Furthermore, the use of a surrounding potting compound can create long-term measurement errors caused by aging and by cracking due to differential thermal expansion between the parts of the heated sensor. Accordingly, the highest quality heated sensors have a skin resistance with a low numerical value that remains constant over the long term.
Stem conduction (in units of watts) causes a fraction of the electrical power supplied to the encased heated sensor to be passed through the stem of the heated sensor, down the housing, lead wires, and other internal parts of the heated sensor, and ultimately to the exterior of the fluid flow channel. Stem conduction couples the electrical power supplied to the encased heated sensor to the ambient temperature outside the channel. If the ambient temperature decreases, stem conduction increases; if it increases, the conduction decreases. In either case, as ambient temperature changes, stem conduction changes, and measurement errors occur. Similarly, stem conduction is responsible for errors in the encased fluid temperature sensor's measurement because it too is coupled to the ambient temperature.
Further discussion of the operational principles of known immersible thermal mass flow meters, their several configurations, particular advantages, uses, skin resistance, and stem conduction are presented in section 29.2 entitled “Thermal Anemometry” by the lead inventor hereof as presented in The Measurement Instrumentation and Sensors Handbook, as well as U.S. Pat. Nos. 5,880,365; 5,879,082; and 5,780,736, all assigned to Sierra Instruments, Inc., and each incorporated by reference herein in its entirety.
As noted in the referenced material, resistance temperature detectors (RTDs) may be employed in the heated sensor and the fluid temperature sensor, when one is provided. Alternative sensors for either the heated sensor or the fluid temperature sensor include thermocouples, thermopiles, thermistors, micro-machined sensors and semiconductor junction thermometers. RTD sensors are generally recognized as being more accurate and stable than any of these alternatives.
RTD sensors operate on the principle of electrical resistance increasing in accordance with increasing temperature. Wire-wound sensors, thin-film sensors and micro-machined RTD sensors have been used variously in thermal anemometers.
Thin-film RTD (TFRTD) sensors offer an edge in accuracy because they are mass produced using automated production operations, employing technologies such as photolithography and lasers. This results in the comparatively high reproducibility, accuracy, stability, and cost-effectiveness of thin-film RTD sensors. Yet, prior to the teaching offered in U.S. Pat. Nos. 6,971,274 and 7,197,953, each to the lead inventor hereof, no high quality application of the thin-film RTD technology-based thermal anemometer was available for industrial applications.
Some thermal anemometers were available that used thin-film RTDs not entirely encased in a protective housing, with the RTD surfaces directly exposed to the fluid. However, due to the fragility, poorer dimensional tolerances, and the oscillating and turbulent flow around the thin-film RTD body, etc., such devices—standing alone—had only proven suitable for light duty, low-end, low-accuracy/precision requirement applications. Still, there are examples in which a thin-film RTD sensor was encased in the tip of a metallic tube (e.g., 316 stainless steel) sealed at its end and surrounded by a potting compound (e.g., epoxy, ceramic cement, thermal grease, or alumina powder). Yet, sensor fabrication with such potting compounds is inherently irreproducible due to variations in their composition, amount used, insufficient wetting of surfaces, and/or air bubbles. These irreproducibilities, combined with the aforementioned high skin resistance and potential for long-term instability associated with the use of potting compounds, limits the overall accuracy of known thermal anemometers constructed in this manner.
The above-referenced Olin patents offered solutions to provide robust and highly accurate thermal anemometers using thin-film RTD technology. Specifically, in a “dry” heated sensor, an apparatus for use as a mass flow meter in a fluid is provided comprising a metal spacer within a metal shell, the spacer having a cross section defining a circular diameter and a rectangular hole, with the spacer adapted to closely hold a thin-film RTD temperature sensor in the hole and with the spacer closely held in a outer casing or shell. In this assembly, the metal spacer body comprises a powdered metal fabricated piece or a machined metal solid. Thus, the active portion of the heated sensor has no potting compound or other bonding materials adjacent thereto.
While of superb quality, such devices have proven less cost-effective to manufacture that those of the present invention. While, at the same time, those according to the present invention offer the same—and possibly better—performance. As such, the present invention offers a further advance in the art.